The present invention relates to electroluminescent elements which are elements which can convert electrical energy into light and which can be employed in fields such as display devices, flat panel displays, back lights, illumination, interiors, markers, signs, electronic cameras, optical signal generators and the like.
Research into organic thin film electroluminescent elements, which emit light when the electrons injected from the cathode and the holes injected from the anode recombine within an organic fluorescent substance interposed between two electrodes, has been actively pursued in recent years. Such elements are receiving considerable attention in that they are of a thin shape, emit light of high luminance under low driving voltages, and provide light emissions of many colours depending on the particular fluorescent material selected. Since the disclosure by C. W. Tang et al of the Eastman Kodak Co. that organic thin film electroluminescent elements emit light of high luminance (Appl. Phys. Lett. 51 (12) 21, p.913, 1987), such research has been carried out by many research bodies. In the typical structure of organic thin layer electroluminescent element presented by the Eastman Kodak Co. research group, there are provided in turn, on an ITO glass substrate, a hole transporting diamine compound, tris(8-quinolinolato) aluminium complex as the light emitting layer, and Mg:Ag as the cathode, and green coloured light emission of 1000 cd/m2 is possible at a driving voltage of about 10 V. At present, some organic thin film electroluminescent elements differ in structure in that, as well as the aforesaid structural components of the element, an electron transporting layer is also provided, but basically the Eastman Kodak Co. structure is still followed.
Amongst the many colours of light emission, research into red coloured light emission as a useful light emission colour is well advanced. Known red coloured luminescent materials include those of the perylene derivatives such as bis(diisopropylphenyl)perylene, the perynone derivatives, the porphyrin derivatives and Eu complexes (Chem. Lett., 1267 (1991)).
Again, as a means for obtaining red light emission, methods have also been investigated for mixing a minute amount of a red fluorescent material in the host material, as a dopant material. Examples of the host material are tris(8-quinolinolato)aluminium complex, bis(10-benzoquinolinolato)beryllium complex, diarylbutadiene derivatives, stilbene derivatives, benzoxazole derivatives, benzothiazole derivatives and the like, and by having present therein, as a dopant material, 4-(dicyanomethylene)-2-methyl-6-(p-dimethylaminostyryl)-4H-pyran, a metal phthalocyanine derivative (MgPc, AlPcCl or the like), a squarilium derivative or a violanthrone derivative, red luminescence is obtained.
However, in the case of the conventional red coloured luminescent materials (host material and dopant material), even where the peak emission wavelength exceeds 580 nm the peak width is broad, so the colour purity has been poor and it has not been possible to obtain a beautiful red coloured luminescence. Moreover, while the rare earth complexes such as Eu complexes have a narrow peak emission width and a beautiful red coloured luminescence is obtained, their maximum luminance is low, ranging from a few to a few tens of cd/m2, so there has been the problem that clear display is not possible.
The present invention has the objective of resolving such problems and offering red electroluminescent elements of high colour purity.